In the last post in this series, we had arrived, at last, at the origins of our very own species, Homo sapiens. Based on the oldest-known remains in the fossil record, we know that anatomically modern humans were present in Africa at about 200,000 years ago. From this starting point, our species was poised to expand its range into Asia and Europe, starting around 100 KYA (i.e 100 kiloyears ago, a shorthand for 100,000 years ago), and in significant measure from about 50 KYA. In doing so, we would follow, and later encounter, other Homo species that had left Africa before us. Such groups include the Neanderthals and Denisovans, as well as Homo erectus, which, as we have seen, also left Africa and was distributed widely in Asia, including the populations in Indonesia that would form the basis for Eugene Dubois’ seminal discoveries. Neanderthals and Denisovans share a common ancestral population at about 400 KYA, though it is not yet clear if their common ancestral population left Africa or if their lineages separated in Africa and both groups migrated out independently. As for Homo erectus, fossil remains establish that it was widely distributed in Asia as far back as 1.8 million years ago.
It is around this time that we reach a point in human evolution that many evangelicals have at least heard about – the last common female ancestor of all modern humans, popularly known as “mitochondrial Eve”, and the male equivalent – our last common male ancestor, popularly known as “Y-chromosome Adam”.
Mitochondrial Eve and Y-chromosome Adam: common ancestors, but not unique ancestors
Wait just a second, you say – isn’t the evidence strong that modern humans descend from a population that has never numbered less than about 10,000 individuals (and as such, is a topic of significant theological consideration)? How is it, then, that all humans can share a single woman and single man as common ancestors? The short answer is that all humans do share a single man and single woman as common ancestors – but that these ancestors are not our unique, or sole, ancestors. Rather, they both come from that population of about 10,000 individuals – the evidence for which (and the theological questions it raises) we will discuss in upcoming posts.
Understanding how humans can have single maternal and paternal ancestors within a genetically diverse population requires us to take a brief excursion into genetics, and specifically how certain forms of DNA are inherited. As we have discussed previously, our mitochondria have their own small chromosome as a remnant of their time as free-living bacteria. In humans, mitochondria are passed down only from mother to child: sperm do not donate mitochondria to the fertilized egg. As a result, mitochondrial DNA is inherited through the maternal lineage only, in contrast to regular chromosomal DNA, which is inherited through both maternal and paternal lineages. The maternal-specific pattern of inheritance for mitochondrial DNA lends itself to certain mitochondrial variants “taking over” a population, which we can illustrate using a large family tree, or pedigree. (A note about pedigree symbols: circles represent females; squares represent males; a horizontal bar connecting them represents a mating; and a vertical bar from a mating is connected to the offspring of that mating.)
In the pedigree below, we see a large extended family that shows the inheritance of three mitochondrial variants (labeled with different colors). In order to keep the pedigree compact enough to show, the dashed lines indicate matings that connect to one another by wrapping around from one side to the other. As we can see, the red “Mito 3” variant has taken over, or “swept” this population. All individuals in the most recent generations of this family share the woman at the top right as a common ancestor for their mitochondria:
Using the same pedigree, let’s now trace some hypothetical Y-chromosome variants. Y chromosomes, obviously, are passed only from father to son, giving a paternal-specific pattern of inheritance. This pattern, like the maternal-specific pattern for mitochondrial inheritance, can also lead to certain variants easily sweeping a population. Let’s suppose that this same family also has three Y-chromosome variants in the oldest generations:
In this case, the “Y chromosome 1” variant sweeps the population, and everyone in the most recent generations has the man highlighted in yellow as their most recent, common male ancestor.
Now that we have identified a common female and male ancestor of the most recent generations in this pedigree, we can illustrate that they are not their unique ancestors. Both the mitochondrial “eve” and Y-chromosome “adam” of this family come from a larger population – and we can easily show this by looking at variation present on regular chromosomal DNA – the kind passed down through both maternal and paternal lineages.
Let’s return to the exact same pedigree, but now illustrate variation on regular chromosomal DNA with different colors. It is now much harder for this variation to sweep a population in short order, because this variation can be passed on by both males and females:
In contrast to the patterns for mitochondrial and Y-chromosome DNA, we see a diversity of regular chromosomal DNA variation transmitted from the oldest generations down to the most recent ones. For example, consider the middle couple in the first generation. While their mitochondrial and Y-chromosome variation has been lost from this population, the regular chromosomal variation of the male (represented as the blue line) has come down to the present day without trouble. As such, we have a “record” of his ancestry in the population, even after his Y-chromosome variation was lost. Similarly, consider the female in the left couple in the first generation. Though her mitochondrial variation was lost, her regular chromosomal variation (represented as the red line) has been passed down. The total amount of genetic variation on regular chromosomes thus is a tool for determining how many ancestors this population has.
It is this variation in regular chromosomal DNA that indicates that this population has not undergone a drastic reduction in population size in the recent past – and that, even though we can point to recent common ancestors for mitochondrial DNA and Y chromosomes, these common ancestors came from a genetically diverse population. So too with our own lineage – we too have a common maternal ancestor of our mitochondrial DNA (“mitochondrial Eve”), as well as a common paternal ancestor for Y-chromosome DNA (“Y-chromosome Adam”). The diversity of our regular chromosomal DNA, however, shows us that these individuals were part of a large, genetically-diverse population. As in the example we’ve worked with, we know this because of the diversity of regular chromosomal DNA we see in modern human populations.
So, why the excitement over these two individuals? In many ways it’s overblown. These individuals are notable solely for being the last common ancestors of only one small part of our genomes (mitochondrial and Y-chromosome DNA, respectively). While an interesting fact, they were not noticeably different from others in their respective populations. If scientists had not labeled them with names alluding to the biblical narrative, they would likely be little-known among Christians.
Locating Mitochondrial Eve and Y-chromosome Adam in time
Current estimates place mitochondrial Eve just after the dawn of Homo sapiens as recorded in the fossil record, at about 180 KYA. This places her within our species. Until recently, Y-chromosome Adam was dated later, at about 50 KYA, the time of significant human migration out of Africa. Recently, however, a rare Y-chromosome variant has been found in modern humans that pushes back the last common ancestor of all human Y-chromosome DNA to approximately 210 KYA - which, interestingly enough, is right at the cusp of our own species as recorded in the fossil record. Since our species arose as a continuous population that gradually diverged from other hominins, there is no reason to expect that all of our DNA variation will come back to a common ancestor (or coalesce, to use the technical term) within our species. Indeed, some of our regular chromosomal variation does not coalesce within our species or even as far back as our common ancestral population with chimpanzees. As we have discussed before, “species” is a term of convenience that biologists use to attempt to draw a line on what is in fact a gradient of gradual change – and biologically, our species is no exception.
In the next post in this series, we will further explore the fuzzy boundaries of our own species as we travel with some of our ancestors out of Africa – and encounter other hominin species in the process.